For the time base conversion, e.g., time base compression or time base expansion, of digital signals, there is a method to use the interpolation processing. This is a method to convert the time base by changing the sampling cycle after converting the number of samples in advance by the interpolation processing so that the sampling cycle after the time base conversion agrees with that before the time base conversion.
Equipment using this technique is known for example in the U.S. Pat. No. 4,623,922 issued for Gerhard Wischermann.
The equipment described in the U.S. Patent is in such structure that interpolators are provided at the input and output sides of a memory.
In this equipment, the time base expansion is made using the memory and the interpolators at its output side. That is, input digital signals are written in the memory by the continuous first clock. The data written in the memory are read by the second clock. This second clock has the same repetition frequency as that of the first clock and becomes discontinued a clock whose pulses lack a fixed cycle. The data read out of the memory is output as a digital signal whose time base is expanded by the operation of the interpolator at the output side.
On the other hand, the time base compression is made using the memory and the interpolator at provided at the memory's input side. That is, input data are written into the memory according to the second clock after the operation by the interpolator at the input side. The data written in the memory are read according to the first clock. Thus, a digital signal with a compressed time base is obtained.
The operation described above will be explained referring to FIGS. 1A to 1G and FIG. 2 by taking a case to extend the time base by 4/3 times as an example.
FIGS. 1A to 1G are timing charts showing the operation when the time base is extended by 4/3 times. FIG. 2 is a circuit diagram showing the outline of the time base expansion system of the above equipment.
FIG. 1A shows the input digital signal whose time base is to be extended. The sampling clock of this signal is a continuous pulse train as shown in FIG. 1A.
FIG. 1B shows the signal in FIG. 1A after extending the time base by 4/3 times. However, this time base expansion extends the sampling period while keeping the data unchanged and the signal is considered to be a virtual sampling train.
By performing the interpolation shown in FIG. 1C using the signal shown in FIG. 1B, a digital signal which has a sampling period the same as that of the signal shown in FIG. 1A and which has its time base extended by 4/3 times is obtained.
The time base expansion processing described above is explained according to FIG. 2.
The signal input from the input terminal 11 as shown in FIG. 1A is written into the memory 12 according to the write clock CK1 that is output from the control circuit 13. This data written into the memory 12 is read out according to the read clock CK2 that is output from the control circuit 13.
The write clock CK1 has the same frequency as that of the sampling clock shown in FIG. 1D. On the other hand, the read clock CK2 has the same frequency as that of the write clock CK1 but is a discontinued pulse train which lacks pulses once every four cycles.
The data x read from the memory 12 by the read clock CK2 shown in FIG. 1E becomes as shown in FIG. 1F. The interpolator 14 at the output side to which this data x is supplied has a register which is operated by the same clock as the read clock CK2. Output of this register is shown in FIG. 1G.
By performing the interpolation shown in FIG. 1C using the adjacent sampling data shown in FIGS. 1F and 1G, the interpolator 14 outputs the digital data having the same sampling period as that of the input digital signal and the time base extended to 4/3 times to the output terminal 15.
According to the conventional equipment described above, the time base of an input digital signal can be converted using a clock of a single frequency without changing the sampling period.
However, the following problems are found in the conventional equipment.
First, in case of the conventional equipment, sufficient frequency characteristic is not obtainable. That is, in case of the conventional equipment, the interpolation is performed using two adjacent sampling data. Therefore, it is considered that the interpolator operates as a low-pass filter composed of two taps (hereinafter referred to as LPF) in the conventional equipment.
However, the frequency characteristic of an LPF in two tap structure has a very gentle-sloping attenuation characteristic. Therefore, this LPF in two tap structure has a problem that the high-pass frequency even in the passing band is attenuated.
For instance, when the time base conversion of TV signal having a 4 MHz band is considered, 14.3 MHz is often adopted as sampling frequency. In this case, if the interpolating sample position is just at the middle point of the input sample position, gain A of LPF in two tap structure will become as shown by the following expression; EQU A=[(1+cos 2.pi.f.multidot.(1/4fsc)/2].sup.1/2
where, Fsc represents the color subcarrier frequency.
When the gain A is expressed as shown above, if f=4 MHz, the gain A drops to 0.816 (1.8 dB). Therefore, if the time base of the TV signal is converted by the conventional equipment, image definition drops.
Further, in case of LPF in two tap structure, as the frequency characteristic is not of phase linear type, unnecessary ringing is generated. Therefore, it is not desirable for image quality to use the conventional equipment for the time base conversion of TV signal.
Further, to perform the time base conversion while keeping the signal band flat, it is necessary to make the interpolator in multiple tap structure. However, the conventional equipment is in such structure that the time base conversion is made using adjacent sampling data, it is not feasible to make the interpolator in multiple tap structure.
In addition, in case of the conventional equipment, if a low-speed operating dynamic memory which is advantageous from the viewpoint of cost is used, a circuit for converting serial data into parallel data is needed.
However, the operating speed of dynamic memory is slow.
In the conventional equipment, however, the memory must be drive at the input digital signal sampling period. Therefore, if this sampling period is short, the memory must be driven at an extremely fast speed and a speed conversion circuit becomes additionally necessary.
Further, in the case of the conventional equipment, it is necessary to provide a first interpolator for the time base compression and a second for the time base expansion, separately.
As described above, the conventional time base conversion apparatus has such problems that sufficient frequency characteristic cannot be obtained, a new data rate conversion circuit becomes necessary when a dynamic memory is used, and it is necessary to provide an interpolating filter for the time base compression and the time base expansion, respectively.